An optical disk has the essential features that the recording medium (disk) can be removed from the recording/playback device, and the recording medium is economical to manufacture. It is therefore desirable that in an optical disk device, high speed/high density recording can be performed without sacrificing these features.
In order to increase the effective recording density (effective surface density) of an optical disk, it is desirable to employ multilayering which makes use of the far-reaching properties and penetration properties of light. However, in the case of three or more layers, there is a trade-off relationship between the transmittance and recording sensitivity of each layer, and either playback signal quality or recording sensitivity has to be sacrificed.
Techniques have been developed to avoid this trade-off. For example in Patent document 1, a recording medium using an electrochromic material comprises plural layers, a voltage is applied to the recording layer between a pair of terminals to selectively color the recording layer, and information is then recorded by irradiating with laser light.
In Patent document 2, an optical recording medium is described wherein both sides of an electrochromic layer are sandwiched by electrically conductive layers. This document however does not discuss multilayer technology, but materials.
In addition to the above, recording in three dimensions including the thickness direction in a transparent organic material, is also known, but in devices which use two-photon absorption, it was not easy to avoid light scattering by a recording mark in a light-incident-side layer during read. If photopolymerization is used for recording, storage stability and recording sensitivity are poor. On the other hand, in hologram recording, a precise laser wavefront is required, so it is difficult to maintain disk compatibility. Further, a large dynamic range is required of the recording medium, a shielding disk case is required and the system is easily affected by coherent noise.
On the other hand, experimental results have been reported to the effect that when a photoconductor and phase change recording film are sandwiched by transparent electrodes, and the assembly is irradiated by light while applying a voltage with these transparent electrodes, the photocurrent is multiplied so that recording can be performed with a laser light which is two orders of magnitude weaker than when only light irradiation is performed (Patent document 4, Non-patent document 1). This may be suitable for high speed recording, but due to the surface resistance of the transparent electrodes, the size and shape of the recording mark easily changes depending on the position on the disk.
A technique is reported in Patent documents 5, 6 and 7 wherein a spherical aberration correction is performed by inserting a stepped glass plate between a focusing lens and the multilayer disk.
Also, a technique is reported in Patent document 8 wherein a first light source is used for focusing control, and a second light source is used for tracking control.
Patent document 1: JP-A No. 346378/2003
Patent document 2: JP-A No. 82360/2002
Patent document 4: U.S. Pat. No. 3,801,966
Patent document 5: JP-A No. 151591/1993
Patent document 6: JP-A No. 151609/1993
Patent document 7: JP-A No. 54740/1991
Patent document 8: JP-A No. 067939/2003
Non-patent document 1: M. Terao, H. Yamamoto and E. Maruyama: Highly Sensitive Amorphous Optical Memory: supplement to the Journal of the Japan Society of Applied Physics, Vol. 42, pp 233-238
Non-patent document 2: Advanced Functional Materials, Vol. 12, No. 2, pp 89-94 (February 2002), Helmut W. Heuer et al.